Optical pyrometer



D. c. GALL 1,820,219

OPTICAL PYROMETER Filed June 2s. 192e Patent-ed Aug. 25, 1931 UNITED STATES DOUGLAS CRISP GALL, OF SOUTH NORWOOD, ENGLAND e 'OPTICAL PYROMETER Application led June 23, 1928, Serial No. 287,683, and in Great Britain-.Tune 29, 1927.

My invention relates to optical pyrometers of the type in which the brightnessof the object whose temperature is to be measured is matched against that of the central portion of the filament of an electric lamp, the brightness of which is adjustable by varying the current through it.

The brightness or temperature scalel of the pyrometer can be defined, if desired, by refer-- ence to the current passing through the lamp filament, but for certain purposes, as is well 1known in the art, it is convenient to employ the total resistance of the lamp lilament or a combination of the current and the total resistance as the variable for refining the temperature scale.

When the total resistance of the filament is so employed, either wholly or partially, certain-'errors are liable to arise, which it is the object ofmy invention to prevent.

In particular, when the temperature of the filament is altered widely, heat is conducted to or from the heavy current leads passing through the base of the lamp. Owing to the mass of these leads and of the materials of the lamp base with which they are in contact, a considerable time elapses before those portions of the filament nearest to the leads attain their equilibrium of temperature and hence of resistance. The readings given before the equilibrium is reached .are thus subject to error.

Another source is that due to change in the temperature of the surroundings of the filament, such as the metal casing in which the pyrometer lamp is contained and which may become considerably heated on exposure in front of a furnace,

It is well known, fromthe laws governing the radiation of heat, that the temperature and resistance of an incandescent filament in a vacuum are not appreciably affected by such changes in the temperature of the surroundings as are likely tob'e encountered 1n practice. This applies to the centralportlon of the pyrometer filament but with the pro-A gressive fall in temperature which occurs in passing from the central portion to the ends of the filament, changes in the temperature "5 0 of the surroundings produce an increasing effect on the temperature and resistance of the filament. .v

In my invention I arrange for the partial or complete elimination of these effects. For this purpose I provide inside the lamp a resistance of appropriate dimensions and disposition preferably wire. The resistance, eiicient of resistance, of this loop are such that they compensate for the leads and the 60 portions of the iilamen connecting the leads with the central portion of the filament. In one arrangement of the compensating loop, I make a loop of wire of thensame material .and diameter as that forming the lainent 05 and p'lace the loop inside the bulb of the lamp where it is subjected-to the same heating or cooling conditions which apply to the filament itself. The loop is in thls arrangement entirely separate from the filament and has therefore two separate leads in the base of the lamp by means of which it may be connected to the circuit in the required manner.

In another arrangement .the loop is partly arranged as a loop of and temperature comerged in the filament of the lamp so that 75 it has only one separate lead in the base.

In aothird construction two compensating loops are used but each compensating loop is partly merged in the filament so that only two additional leads are required.

Various embodiments of the invention will now be described with reference to the accompanying drawings in which: 1

Fig. l shows a lamp having two compensating loops each partly merged in the filament of the lamp and each havin only one additional lead in the base of t e lamp.

Fig. 2 shows a known Wheatstone bridge circuit arrangement with a lamp of ,known type.

Fig. 3 shows a lamp having an entirely separate compensating loop wlth .two separate leads in the base. v f

Fig. 4; shows the preferred Wheatstone bridge arrangement and the position of the .lamp filament and the compensating loop in lcontiguous arms of the bridge.

Fig. 5 shows a third construction of the lamp in which one compensating loop is used which is partly merged in the lamp lament and has therefore only one separate lead in the base and a circuit arrangement for using the lamp.

Fig. 6 showsa known circuit arrangement in which a differential galvanometer'windingvand a known lamp are used.

ig. 7 shows the application of a lamp according to Fig. 3 to the circuit according to Fig. 6.

Fig. 8 shows the application of a lamp according to Fig. 5 to the circuit according to Fig. 6.

When the voltage drop on the lamp as given, by a voltmeter, is used for the purose of the temperature scale, I arrange the amp filament as shown in Fig. 1. Here a and a represent the usual leads for the current passing through m the glass envelope of the lamp, but instead of attaching the voltmeter to a and a I attach it to the leads b, b which are connected by very line wires to the lilament at points p, p. These latter points are so chosen that the resistance of the portion of the filament lying between them isnot, as explainedabove, appreciably adected by changes in the temperature of the surroundings. For the same current, therefore, a constant voltage drop is always obtained between the points p, p. Moreover the errors which, as explained above, arise from the lag in `the temperature of the base of the lamp and the heavy leading-,in wires now become unimportant since with the arrangement shown in Fig. 1, the leading-in wires b, b and the wires connecting b, b to p, p form part of the Voltmeter circuit and normally account for only a small fraction of the total resistance of that circuit.

When the resistance only of the lamp is used asvthe characteristic for temperature measurement, the lamp is associated with some form of resistance measuring device, such for example, as the Wheatstone bridge shown in Fig. 2. Here K is the lamp c, d, e and a: fixed resistances, g a galvanometer connected to the resistance by a movable contact which can be moved along until the bridge is lbalanced and no current flows through the galvanometer. S is a battery and 1' a rheostat, for supplying and regulating the current to the bridge and the lamp,

. and hence used or varying the temperature of the lamp filament.

When the bridge shown in Fig. 2 is used as 'above indicated, the resistance characteristic of the lamp is employed, for the purpose of the temperature scale.

If, however,.the galvanometer contact with ab is fixed and the resistance of the arms of the bridge are such that for a certain current through the bridge it is balanced, while for an increased or decreased current the resistance of the lamp K changes relatively to that i of the other )arms of the bridge, which is thus thrown out of balance. Then the resulting current through the galvanometer g can be used for the purpose of obtaining the temperature scale. The scale in this case would depend on a combination of the resistance and current characteristics of the lamp. In either of the above arrangements I compensate for the errors already described by means of a resistance of appropriate dimensions and dispositiomwhich may be conveniently arranged as a loop of wire, as shown in Fig. 3. Here a, a represent, as in Fig. 1, the thick current leads passing through the glass envelope of the lamp and, connected to the filament, and f, f are similar leads connected to a loop of wire. The resistance, and temperature coeficient of resistance, of this loop are such that they compensate for the portions of the filament which are liable to e affected in resistance by changes in the temperature of the surroundings. The circuit arrangement for this purpose may conveniently be as shown in Fig.4. As in Fig. 2 c and d are fixed resistances, or ratio arms, which may conveniently be made equal while the lamp filament with vthe current leads a, a form, wholly-or in part, another arm of the bridge and the compensating loop with the leads f, f form, wholly -or in part, the ref maining arm of the bridge. It is obvious that, with the arrangement shown, the two sources of error to which attention has been drawn above, apply equally in contiguous arms of the bridge and hence compensate for each other.

In order to adjust the magnitude of the eiect produced by the compensating loop, the

form the whole of the arm of the showing a circuit arrangement suitable for this case is given'in Fig. 5. In this arrangement c and d are ratio arms which would normally be equa-l in resistance. The lamp being connected to the bridge as shown, it will vbe noted that the two current leads a, a are situated in different arms of the bridge and hence the errors due to temperature lag in these leads and the lamp base balance each other. Further, there are `similar portions of filament mp in each arm, which will be aected equally by changes in the tempera.

ture of the surroundings, and hence compensate for each other. As in the case of the previous arrangement, the loop shown as b, p, a in Fig. 5 may be shunted by a loop of different material or the whole of the arm oi Sis which b, p, a forms a part may be so shunted. In the case of Figs. 2, 4 and 5 the battery and galvanometer may be interchanged if desired. A

Fig. 6 shows a known circuit arrangement for optical pyrometers in which a differential galvanometer winding and a known pyromi eter lamp are used. In this figure 1 and 3 are resistances, K is the pyrometer'lamp, g1 and g2 are the two coils of a differential galvanometer. These two coils are wound on 1 the same bobbin and can rotate between the poles of the usual galvanometer magnet. The currents in g1 and g2 can be arranged to be in opposite directions, and then by adjusting the resistance 4 the galvanorneter can be made to give no deection for any desired temperature or resistance) of the lamp (say 700 0.). 2 is another adjustable resistance, a source of current and 1' is a rheostat. On increasing the current through the bridge, the resistance of the lamp is altered while 4 remains the same in resistance and hence a defiection is caused in the galvanometer owing to the different currents in the coils g1 and g2. This arrangement is subject to precisely the same errors as the previously described Wheatstone bridge arrangement, and these errors can be eliminated or reduced in precisely the same way by including a compensating resistance in the arm 4.

Fig. 7 shows the lamp according to Fig. 3 applied to the circuit of Fig. 6, the compensating resistance being indicated at f, fand the lamp filament at a, a.

- Fig. 8 shows the employment of the lamp of Fig. 5 to the circuit of Fig. 6. It will be seen that the main leads a, a are connected to two arms 4 and 5 whereas the additional lead is connected to the resistance 2 and the point p of the filament.

While I have described above certain examples of circuits to which my invention is applicable, it will be understood that there are many other arrangements of circuits to which it would also apply. The essential feature of my invention is that by means of apyrometer lamp of one of the typesldescribed with compensating resistance inside the lamp bulb it is possible to insure that, in edect only the central portion of the filament, which is immune from errors due to changes in temperature of the surroundings or lag edects in the base of the lamp or leading-in wires, is

. combination:

used for the perature scale.

I claim:

l. An optical pyrometer comprising in a Wheatstone bridge, a galvanometer in said bridge, a lamp in one ot the arms of the bridge, a compensating resistance loop inside said lamp arranged in a contiguous arm of the bridge, a source of current for supplying current to the bridge purpose or obtaining the tem-n and tothe lamp and means fon regulating the current. j

2. An optical pryometer comprising a Wheatstone bridge, a galvanometer in said bridge and ixedly connected to opposite points of the bridge, a pyrometer lamp in one of the arms of the bridge, a compensating resistance in said lamp arranged in av contiguous arm of the bridge, a source of current for supplying current to the bridfre and the lamp, and means for regulating thecurrent substantially as described.-

3. In an optical pyrometer of the type in a variable source of electric current; a

filament portions connecting the central porv tion to the leads, all arranged in one branch of the network; a compensating resistance contained within the lamp arran ed in another branch o the network so c osen that an increase in resistance of this branch acts on the galvanometer in an opposite sense to an increase in resistance of the branch containing the ilament, the said lament and compensating resistance being the only parts of the network, the resistances of whlch are substantially affected by changes of the electric current passing through them, substantially as described.

4. In an optical pyrometer of the .type in which the brlghtness of the object to be measured is matched against that of the central portion of the' ilament of an electric lamp, the brightness of which is adjustable by varying the current through it, the combination of: a variable source of electriccnrrent; a network of resistances connected at two Vpoints with the source of electric current; a galvanometer connected with the network of resistances to indicate any change in the resitance of any one branch of I the network; a

ioo

tion and a compensating resistance inside the lamp having a common lead connected to one end of each and having separate leads connected to the other two ends, the three leads passing out through the base of the lamp and being so connected to the network that the filament and compensating resistance are in conti nous branches of the network, the said i amont and compensating resistance being the only parts of the network the resistances of which are substantially afieeted by changes of the electric current passing a central filament portion,

ing through them, substantially as described.

5. In anpptical pyrometer of the type in which the brightness of the object to be measured is matched against that of the central portion of the lament of an electric lamp, the brightness of which is adjustable by varying the current through it, the combination of: an electric circuit having four arms, a bridge connecting two diagonal points of the circuit, a variable source of current connecting the twofother diagonal points of the circuit, a pyrometer lamp in said circuit; means 'for altering the current passing through the lamp; a galvanometer in said circuit by the aid of which the temperature of said lamp is measured; said lamp comprisleads throuvh the base, filament portions connecting the central portion to the leads, al1 arranged in one arm of the circuit, and a compensating resistance inside the lamp arranged in a contiguous arm of the circuit, substantially as described.

Signed at London this 13th 1928.

day of dune,

DOUGLAS CRISP GALL. 

